Ion implantation is a physical process, as opposed to diffusion, which is a chemical process, employed in semiconductor device fabrication to selectively implant dopant into semiconductor and/or wafer material. Thus, the act of implanting does not rely on a chemical interaction between a dopant and the semiconductor material. For ion implantation, dopant atoms/molecules are ionized and isolated, sometimes accelerated or decelerated, formed into a beam, and swept across a wafer. The dopant ions physically bombard the wafer, enter the surface and come to rest below the surface.
An ion implantation system is a collection of sophisticated subsystems, each performing a specific action on the dopant ions. Dopant elements, in gas or solid form, are positioned inside an ionization chamber and ionized by a suitable ionization process. In one exemplary process, the chamber is maintained at a low pressure (vacuum). A filament is located within the chamber and is heated to the point where electrons are created from the filament source. The negatively charged electrons are attracted to an oppositely charged anode also within the chamber. During the travel from the filament to the anode, the electrons collide with the dopant source elements (e.g., molecules or atoms) and create a host of positively charged ions from the elements in the molecule.
Generally, other positive ions are created in addition to desired dopant ions. The desired dopant ions are selected from the ions by a process referred to as analyzing, mass analyzing, selection, or ion separation. Selection is accomplished utilizing a mass analyzer that creates a magnetic field through which ions from the ionization chamber travel. The ions leave the ionization chamber at relatively high speeds and are bent into an arc by the magnetic field. The radius of the arc is dictated by the mass of individual ions, speed, and the strength of the magnetic field. An exit of the analyzer permits only one species of ions, the desired dopant ions, to exit the mass analyzer.
An acceleration system, referred to as a linear accelerator, is employed in some instances to accelerate or decelerate the desired dopant ions to a predetermined momentum (e.g., mass of an dopant ion multiplied by its velocity) to penetrate the wafer surface. For acceleration, the system is generally of a linear design with annular powered electrodes and pairs of quadruple lenses along its axis. The quadruple lenses are powered by negative and positive electrical potentials. As the dopant ions enter therein, they are accelerated therethrough by the powered electrodes and are (as a beam) selectively focused and defocused by the quadruple lenses.
Continuing on, the dopant ions are then directed towards a target wafer at an end station. The dopant ions, as a beam, impact the wafer with a beam intensity, which is a measure of the number of particles per unit time as a function of position, and emittance, which is an angular distribution (angle of incidence) of the beam as a function of position. Generally, it is desirable that the beam intensity and emittance be substantially uniform and at expected or desired values.